86 BIRDS OF LA PLATA 



and of the Spine-tail, Synallaxis hudsoni. Yet these 

 birds are beautifully adapted in structure, coloration, 

 and habits to their station. It thus happens that in 

 districts where the Molothrus is abundant, their eggs 

 are found in a majority of the Pipits* nests ; and 

 yet to find a young Cow-bird out of the nest is a 

 rare thing here, for as soon as the young birds are 

 able to quit the nest and expose themselves they are 

 mostly carried off by the Hawk, 



CONJECTURES AS TO THE ORIGIN OF THE PARASITIC 

 INSTINCT IN M, bonariensis 



Darwin's opinion that the " immediate and final 

 cause of the Cuckoo's instinct is that she lays her 

 eggs not daily, but at intervals of two or three days " 

 (Origin of Species) carries no great appearance of 

 probability with it ; for might it not just as reasonably 

 be said that the parasitic instinct is the immediate 

 and final cause of her laying her eggs at long inter- 

 vals i If it is favourable to a species with the instinct 

 of the Cuckoo (and it probably is favourable) to lay 

 eggs at longer intervals than other species, then 

 natural selection would avail itself of every modifi- 

 cation in the reproductive organs that tended to 

 produce such a result, and make the improved 

 structure permanent. It is said (Origin of Species, 

 chapter vii.) that the American Cuckoo lays also at 

 long intervals, and has eggs and young at the same 

 time in its nest, a circumstance manifestly disadvan- 



